extensionSystem: Always enforce disallowing extensions using sessionMode

extensionSystem: Always enforce disallowing extensions using sessionMode

It's currently possible to circumvent the `sessionMode.allowExtensions`
property: For already enabled extensions one can call reloadExtension
via DBus, for new extensions it's possible by adding the extension to
the enabled-extensions gsettings key and setting the
disable-extension-version-validation key (which triggers a reload of
`this._enabledExtensions`) and then calling reloadExtension via DBus.

So to enforce `allowExtensions` while still allowing to update
extensions and keeping the extensionSystem synced with various gsettings
keys, replace the checks for `this._enabled` with simple checks for
`Main.sessionMode.allowExtensions` inside `_callExtensionInit()` and
`_callExtensionEnable()`.

The remaining checks for `this._enabled` are only small optimizations to
prevent running code on irrelevant sessionMode updates.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/96